There is a case where some of electronic devices are standing by to be ready to return to the normal operation state. In such a standby state, power is hardly consumed. There is a case where a power supply which supplies power to an electronic device as a load should be stopped if power is not consumed, from viewpoints of reduction in power consumption and noise.
Such a power supply, which determines that a loadless state has come when the current to be supplied to a load becomes smaller, and stops supplying the input voltage to achieve low power consumption, is disclosed in, for example, Unexamined Japanese Patent Application KOKAI Publication No. H2-294267.
If the power supply is not activated, however, the current does not flow in the load, which is the same as the loadless state. Once such a conventional power supply goes into a standby state, therefore, the power supply will never be activated unless an activation signal to activate the power supply is supplied from outside. In other words, the conventional power supply should be supplied with the activation signal from outside, and such a method cannot be adapted to one which cannot be supplied with the activation signal from outside.